Oil and gas field pumping units conventionally convert a rotary motion from an electric or gas powered engine to a vertical reciprocating motion for moving a subsurface pump and sucker rods in a tubing string for vertically removing liquid from an oil, gas, or water bearing formation. The subsurface pumps typically employ a series of lift check valves within a tubing string to cause vertical movement of liquid within the tubing string. But the check valves seal against and move relative to the tubing string so that there is substantial wear of the down hole components. This wear is increased when a tubing string and associated cased well bore are not perfectly vertical, but have significant amounts of deviation from vertical, i.e., the casing is “crooked”.
In an oil and gas field, the fluid level in the casing-tubing annulus must be maintained at some minimum depth in order to reduce the hydrostatic head of the fluid in the casing-tubing string and enable the oil, gas, and water to enter the casing. Typically, the subsurface pump is sized to pump more volume of liquid than will enter the well bore over time so that a pump does not have to pump continuously to maintain a selected fluid level between selected elevations, i.e., to maintain a selected maximum hydrostatic head. Thus, continuous pumping unnecessarily aggravates wear in the surface and down hole pumping unit system components.
It will be appreciated that replacing down hole components as a result of wear is expensive and time consuming since the entire pump string must be removed and refurbished. For example, if the duty cycle of a pumping unit is reduced by a factor of four, the replacement cycle period for down hole components is increased by a factor of four with a substantial reduction in costs and increase in well utilization.
Pumping units typically may be powered by electric motors or by natural gas powered engines. Where electric motors are used, the motor may be simply turned on and off according to a predetermined cycle to control the pumping cycle and concomitant liquid level. But in remote locations where engines are used, it is not desirable to turn the engines on and off because of reliability problems, reduced battery life under repeated start cycles, and the labor needed to periodically return to a pump site. Until the present invention, there has not been a suitable control system for providing a reliable duty cycle from pumps using natural gas engines.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.